Electrical power generation systems powered by variable speed prime movers that require highly regulated electrical output, such as electrical power generation systems used for aeronautical applications, generally use a wound field synchronous machine (WFSM) that serves as an electrical generator. This is because it is easy to adjust rotor current to regulate electrical output of a WFSM. In aeronautical applications, the prime mover is often a gas turbine engine that has a normal rotational velocity that exceeds 20,000 revolutions per minute (rpm). Due to the rotational velocity limitations of the WFSM, such electrical power generation systems generally require a reduction gearbox between the prime mover and the WFSM. This increases weight, cost and complexity of the electrical power generation systems.
Electrical power generation systems may alternatively employ an electrical machine of the permanent magnet type as an electrical generator. Such a permanent magnet machine (PMM) is capable of much higher rotational velocity than a WRSM of similar output and therefore it is capable of direct coupling to the prime mover, thereby eliminating the reduction gearbox. This results in reduced weight, cost and complexity of an electrical power generation system. However, traditional PMMs have no convenient means to alter magnetic flux for regulating their output.
Especially in aeronautical applications, it is desirable to utilize the electrical power generating system as a starting system for the prime mover. This reduces weight and bulk compared to utilizing separate starting and generating systems and it has the potential for reducing overall cost as well. The starting system may require a sensorless motor control that employs an open loop mode during start up to at least some minimum speed and a closed loop mode above this minimum speed.